Anti-collision interrogation pulse focusing system for use with multiple surface acoustic wave identification tags and method of operation thereof

ABSTRACT

The present invention provides a system for avoiding code collisions from multiple SAW identification tags and a method of operating such system. In one embodiment the invention provides for (1) focusing an interrogation pulse to within a defined space; and (2) discriminating between coded responses returned from tags located within such defined space.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to a signal focusingsystem and, more specifically, to a system to discriminate betweenmultiple surface acoustic wave (SAW) identification tags by focusing aninterrogation pulse and a method of operation thereof.

BACKGROUND OF THE INVENTION

There are a number of electronic identification methods and devicespresently in use. Everyone is familiar with the ubiquitous bar codes andmagnetic strips that, together with their readers, are widely employedby businesses and others. An inherent limitation of bar codes andmagnetic strip is the effective range at which they can be read, whichis quite short. Magnetic strips, for example, generally require thereader to be in direct contact with the strip in order to detect anddecode any data. In the very few cases where a magnetic strip is readwith a device other than a direct contact reader, the effective readingrange is still only a few centimeters at best. Similarly, the effectiverange at which bar codes can be reliably read is typically no betterthan a few centimeters. Because the read range for bar codes andmagnetic strip is so short, they are usually read one at a time andseldom does one bar code or magnetic strip interfere with another.

Another prior art identification device is the radio frequencyidentification (“RFID”) tag. When interrogated, RFID tags reflect orretransmit a radio frequency signal that returns an encodedidentification number, such as the RFID tags used to collect highway andbridge tolls. Although prior art RFID tags based on a chip have a longerreliable range than magnetic strips or bar codes, they are generally soexpensive that they are not widely used, which means that each prior artRFID tag is generally individually read. In short, very littleopportunity exists for prior art RFID tags to interfere with oneanother.

With the introduction of inexpensive identification tags based onsurface acoustic wave (SAW) technology that can be read at a relativelylong range, certain interference problems must now be addressed.Although there will not be a problem where SAW tags are individuallyread or simultaneously read in small groups, such will not be the casewhere a large number of SAW tags are being simultaneously interrogated.Such would be the case, for example, where a large number of articlesare stacked on a pallet with each article bearing its own globallyunique SAW tag. The large number of responses to an interrogation pulsewould make it difficult for the SAW tag reader to accurately detect anddecode each response in order to reliably identify each article on thepallet. This type of code collision problem as well as the inter-symbolinterference problem caused by so many responses being transmitted atone time needs to be addressed before the full potential using SAWidentification tags can be realized.

Accordingly, what is needed in the art is a system for focusing aninterrogation pulse to within a defined space so that a SAWidentification tag reader can discriminate between coded responsesreturned from SAW tags located within that space.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides a system for avoiding code collisions frommultiple SAW identification tags and a method of operating such system.In one embodiment the invention provides for (1) focusing aninterrogation pulse to within a defined space; and (2) discriminatingbetween coded responses returned from tags located within such definedspace.

The present invention thus provides a system for controlling potentialidentification signal collisions where an interrogation signal cangenerate responses from multiple SAW identification tags. The systemdescribed herein is useful in an environment where several SAWidentification tags simultaneously transmit identification signals inresponse to an interrogation pulse. If a large enough number ofresponses are generated at one time in the allocated frequencybandwidth, a SAW tag reader will have difficulty in accurately detectingand decoding such responses. The present invention limits the number ofpotential responses to a level where the SAW tag reader is able toaccurately detect and decode responses to an interrogation signal.

In one embodiment of the invention the system uses beam steering tofocus the interrogation pulse within the defined space. In anotherembodiment an antenna is used to focus the interrogation pulse withinthe defined space. An aspect of this embodiment provides for an antennathat is a parallel conductor pair. This is a particularly usefulembodiment that permits interrogation while in close proximity to a SAWtag. In still another aspect the antenna is a helical antenna while inyet still another the antenna is a dielectric waveguide antenna.

In another embodiment of the invention a reflector is coupled to theantenna. One aspect of this embodiment provides for an antenna that isan elliptical trough. The elliptical trough is a particularly usefulfocusing device when used with an antenna. In another embodiment of theinvention the reflector is selected from the group consisting of: acircular reflector; a curved reflector; a parabolic reflector; are-entrant cavity; and an elliptical reflector.

In another embodiment of the invention the interrogation pulse isfocused within the defined space by a waveguide. One aspect of thisembodiment provides for a waveguide that projects a circular polarizedbeam. In another aspect of this embodiment the waveguide has a firstfeed of about one-quarter wave length positioned at about a 90° positionrelative to a second feed of about one-quarter wave length. In yetanother embodiment of the invention a reflector is coupled to thewaveguide. A particularly useful aspect of this embodiment provides forthe reflector to be selected from the group consisting of: a circularreflector; a curved reflector; a parabolic reflector; a re-entrantcavity; and an elliptical reflector.

The foregoing has outlined preferred and alternative features of thepresent invention so that those skilled in the art may better understandthe detailed description of the invention that follows. Additionalfeatures of the invention will be described hereinafter that form thesubject of the claims of the invention. Those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiment as a basis for designing or modifying otherstructures for carrying out the same purposes of the present invention.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a representative article handling device, such asthat used to handle parcels or baggage, where an embodiment of a SAWidentification tag reader is being used to identify articles;

FIGS. 2A-2B illustrate a representative embodiment of an antenna arraywhere signal phases delivered to the antennas are shifted to steer thetransmitted interrogation pulse beam to a defined space;

FIG. 3 illustrates an embodiment of an antenna configured as anelliptical trough for focusing an interrogation pulse within a definedspace;

FIG. 4 illustrates an embodiment of a proximity wand reader using anantenna configured as a parallel conducting pair;

FIGS. 5A-5B illustrate an embodiment of a helical antenna and a helicalsignal generating device with two feed inputs for producing a circularpolarized interrogation signal;

FIGS. 6A-6B illustrate embodiments of dielectric waveguide antennastructures for focusing an interrogation pulse;

FIG. 7 illustrates an embodiment of an antenna apparatus with areflector coupled to the antenna;

FIG. 8 illustrates an embodiment of a waveguide used to focus aninterrogation pulse within a defined space;

FIGS. 9A-9B, illustrate embodiments of an interrogation pulse focusingdevice constructed in accordance with the present invention that can beused to launch both right hand and left hand circular polarizedinterrogation pulses; and

FIG. 10 illustrates a SAW tag embodiment where the response to aninterrogation pulse can be turned off.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a representative articlehandling device 100, such as that used to handle parcels or baggage,where an embodiment of a SAW identification tag reader 110 is being usedto identify articles 120. A conveyor belt 130 being used to transportarticles 120 from one location to another has a SAW identification tagreader 110 associated with it in a position where it can transmitinterrogation pulses to generate responses from SAW tags 140 on thearticles 120. Each SAW tag 140 responds to the interrogation pulse bytransmitting its own globally unique coded response. The reader 110detects and decodes these responses and identifies each SAW tag 120 and,inferentially, the parcel 110 to which it is attached.

A complete and detailed description of SAW identification tags 120 isset forth in detail in U.S. patent application Ser. No. [Attorney DocketNo. RFSC-0001], entitled “Surface Acoustic Wave Identification TagHaving Enhanced Data Content and Methods of Operation and ManufactureThereof,” Hartmann, Clinton S. (“Hartmann One”), commonly assigned withthe invention and incorporated herein by reference. A description of SAWidentification tag readers 130 is described in detail in U.S. patentapplication Ser. No. [Attorney Docket No. RFSC-0003], entitled “ReaderFor a High Information Capacity Saw Identification Tag and Method of UseThereof,” Hartmann, Clinton S. (“Hartmann Three”), also commonlyassigned with the invention and incorporated herein by reference.

When a limited number of articles to be identified are being transportedon the conveyor belt 130, the SAW tag reader 110 will most probably beable to isolate and decode each response. Such would be the case, forexample, if the device 100 were a luggage handling apparatus withsuitcases and other luggage items being transported by the conveyor belt130 in more or less of a line so that only one or two SAW tags 140 arebeing interrogated at the same time. However, if the number of articles120 being interrogated is very large, such as would be the case if apallet of articles 120, each with its own SAW tag 140, was beinginterrogated, a correspondingly large number of responses would begenerated. The large number of responses being simultaneously generatedcould make it difficult for the SAW tag reader 110 to accurately detectand decode each response so that each article 120 could be reliablyidentified. This code collision problem is caused by both the largeamount of data required to be detected and processed as well as by theinter-symbol interference generated from so many responses beingtransmitted at one time. The present invention provides a system foravoiding such code collisions as well as a method of operating suchsystem.

The present invention provides a system for focusing an interrogationpulse to within a defined space so the SAW identification tag reader 110can discriminate between coded responses. By using the present inventionto focus an interrogation pulse within a definite space so only SAW tagswithin that space are interrogated, the problem of code collision can bemore easily controlled. Because the SAW tag reader 110 will only receivea limited number of responses to an interrogation pulse, its ability todistinguish between SAW tag 140 responses will be enhanced and thearticles 120 to which SAW tags 140 are attached can be accuratelyidentified.

One embodiment of the invention uses beam steering for focusing theinterrogation pulse within the defined space to permit the SAW tagreader 110 to more easily discriminate between coded responses. Thereare a number of different beam steering techniques known to those ofordinary skill in the pertinent art, all of which are within theintended scope of the present invention. These range from mechanicallysteering the interrogation pulse beam to using an antenna array with asophisticated electronic phase shifting apparatus to steer the beam.

Turning now to FIGS. 2A-2B, illustrated is a representative embodimentof an antenna array 200 where signal phases delivered to the antennas210 are shifted to steer the transmitted interrogation pulse 220 beam toa defined space. The input signal 230 to each antenna 210 in the array200 is routed through an associated phase shifter 240, where the phaseof the signal 230 delivered to the antenna 210 is shifted to steer thetransmitted beam in a predetermined direction. If the phase of a signal230 is not shifted, as illustrated in FIG. 2A, the signals 230 deliveredto each antenna 210 will be in-phase and the interrogation pulse 220will proceed directly away from the array 200. Illustrated in FIG. 2B isan example where the phase of the input signal 230 delivered to eachantenna 210 is changed with the consequent result of redirecting thebeam of the interrogation pulse 220.

An antenna array 200, such as that illustrated in FIGS. 2A and 2B, isespecially useful because it lends itself to being fabricated as aprinted antenna array 200. Using well known lithographic techniques,such printed antennas 210 can easily be integrated into printed circuitswith the necessary electronics to control the phase of the signal toeach antenna 210. Thus it is convenient to incorporate an entire antennaarray 200 module into a SAW tag reader.

Turning now to FIG. 3, illustrated is an embodiment of an antenna 300configured as an elliptical trough 310 for focusing an interrogationpulse within a defined space. Because an ellipse is a closed plane curvegenerated by a point moving in a manner that the sums of the distancesbetween the moving point and two fixed points or foci 320A, 320B is aconstant, an elliptical trough 310 is a particularly useful antennaconfiguration for reading SAW tags 330. When an interrogation signal isfed to an antenna 300 located at the first focal point 320A of theellipse, the transmitted signal reflects and refocuses at the secondfocal point 320B where, optimally, a SAW tag 330 is interrogated. Byusing an elliptical trough 310 with a linear feed 340 extending thelength of the trough 310, a second focal point 320B will also have alength equal to the linear feed 340. This is particularly useful forreading SAW tags 330 in certain environments, such as the articlehandling device 100 illustrated in FIG. 1. Where a SAW tag reader 110has an elliptical trough 310 antenna 300, the second focal point 320Bcan be designed to extend across the width of the conveyor belt 130 sothat each article 120 on the belt 130 is interrogated.

In another embodiment of the invention, the illustrated ellipticaltrough 310 is usefully employed in a cross beam configuration toprecisely focus the interrogation signal and response. For example, oneelliptical trough 310 used to transmit an interrogation signal can bepositioned about perpendicular to another that receives a response, thusprecisely focusing on the specific SAW tag 330 to be identified. Asthose of ordinary skill in the pertinent art will understand, either ofthe elliptical troughs 310 can be programmed to move in a sweepingpattern to provide a precisely focused beam across a field of interest,such as a conveyor belt where the trough 310 positioned across the beltis fixed but the trough 310 paralleling the belt is moved in a sweepingmotion. Those of ordinary skill in the pertinent art will alsounderstand that other configurations of elliptical troughs 310 as wellother embodiments of antennas, waveguides and reflectors can also beused in an intersecting or cross beam focusing arrangement such as thatdescribed and be well within the intended scope of the presentinvention.

Turning now to FIG. 4, illustrated is an embodiment of a balanced feedproximity wand reader 400 using an antenna configured as a parallelconducting pair 410. The wand 400 has a parallel pair of transmissionlines 420 that receive a feed signal 430. The feed signal 430 causes afield to develop around the wand 400 sufficiently strong enough tointerrogate SAW tags by placing the wand 440 in close proximity. Thoseof ordinary skill in the pertinent art will understand that a singlewire antenna can also be used with a wand 400 such as that illustratedand still be within the intended scope of the present invention.

Turning now to FIG. 5A, illustrated is an embodiment of a helicalantenna 500 used to focus an interrogation pulse. The illustratedantenna 500 has a canister 510 around a helical signal generating device520 (FIG. 5B) to which a pulse interrogation signal is delivered by twofeed inputs 530 located at about ninety degrees relative to each other.The helical signal generating device 520 produces a circular polarizedinterrogation signal that can be directed to within a defined space bythe cannister 510 where a SAW tag 540 can be interrogated. Using acircular polarized signal to interrogate SAW tags 540 is particularlyuseful because a pulse can be delivered regardless of the SAW tag 540orientation within a plane normal to the interrogation signal.

Turning now to FIGS. 6A-6B, illustrated are embodiments of dielectricwaveguide antenna structures 600, 605 for focusing an interrogationpulse. The embodiment in FIG. 6B has a handle 606 that permits it to beused as a “wand” to focus interrogation pulses by hand. Each has aground plane 610 with a dielectric waveguide 620 coupled thereto in anapproximate normal position. A feed line 630, generally a coaxial cable,delivers an interrogation signal 640 or pulse to a dipole antenna 650located within the dielectric waveguide 620. The propagation rate of thesignal 640 through the dielectric material will be slower than suchsignal's 640 propagation rate in the air. The propagation rate of thesignal 640 through the waveguide 620 material will also be dependent onthe dielectric constant of such material. Because the distance thesignal 640 must travel through the waveguide 620 before it reaches thesurrounding air is a variable, the signal 640 will not all make thetransition into the air at the same time. The waveguide 620 can beshaped, however, to provide for a significant portion of the propagatedsignal 640 to transition from the dielectric body of the waveguide 620into the surrounding air at or near the same time, thus providing aconcentrated signal 640 that is focused.

Turning now to FIG. 7, illustrated is an embodiment of an antennaapparatus 700 with a reflector 710 coupled to the antenna 720. Thereflector 710 reflects the interrogation signal transmitted by theantenna 720 and focuses such signal to within a defined space withinwhich a SAW tag 730 can be interrogated. Note that the illustratedreflector 710 has an elliptical shape that focuses the interrogationsignal at a second focal point 740 where the SAW tag 730 is located.Although an elliptical shape can be advantageously used in certainsituations, there are other situations calling for differently shapedreflectors. A useful aspect of this embodiment provides for thereflector to be selected from the group consisting of: a circularreflector; a curved reflector; a parabolic reflector; and an ellipticalreflector. Of course, as will be understood by those of ordinary skillin the pertinent art, any reflector shape is within the intended scopeof the present invention.

Turning now to FIG. 8, illustrated is an embodiment of a helical source800 for using circular polarized waves and a focusing antenna to addressa SAW tag. Also illustrated is a re-entrant cavity 810 to providedirectionality for the helical source. Focusing reflectors of the typeillustrated are discussed in more detail above relative to FIG. 7. Ofcourse, as was the case above, a reflector can be selected from thegroup consisting of: a circular reflector; a curved reflector; aparabolic reflector; and an elliptical reflector.

Turning now to FIGS. 9A-9B, illustrated are embodiments of aninterrogation pulse focusing device 900, 950 constructed in accordancewith the present invention that can be used to launch both right handand left hand circular polarized interrogation pulses 930. In FIG. 9A, athin walled closed end cylinder 905 with two inputs 910, 915 is used tolaunch either right hand or left hand circular polarized interrogationpulses 930. The illustrated embodiment is metallic with a closed firstend 920 (not shown) and an open second end 930. The two inputs 910, 915used to launch the circular polarized interrogation pulses 930 arelocated at about one quarter of a wavelength from the closed first end920 and positioned on the circumference of the cylinder 900 at about 90°relative to each other.

To launch a circular polarized pulse from the cylinder 900, a signal isfed to the cylinder 900 via the inputs 910, 915 which are positioned sothey have a 90° degree phase angle delay relative to each other. Forexample, if a left hand polarized pulse is to be launched, the phaseangle of the signal fed to the first port 910 will be a 0° while thephase angle of the signal fed to the second port 915 will be −90°relative to the first port 910 signal. This input produces a TE₁₁electric field with a consequent clockwise rotating e field, thuslaunching a right hand circular polarized pulse 930 that retains itscircular polarity after leaving the device 900. To launch a left handcircular polarized pulse 930, the phase angle of the signal fed to thesecond port 915 would have a +90° phase angle difference with respect tothe phase angle of the signal fed to the first port 910. Those ofordinary skill in the pertinent art will understand that any transverseelectric or transverse magnetic pattern can be produced and be withinthe intended scope of the present invention.

The embodiment illustrated in FIG. 9B functions in the same manner asthat illustrated in FIG. 9A, except a solid dielectric cylinder 950 isused to launch right and left hand polarized pulses. This isparticularly useful embodiment because a thin dielectric structure canbe made that provides for a neat, compact device for launching eitherright hand and left hand circular polarized pulses 230. Of course, adielectric cylinder 950 similar to that illustrated in FIG. 9A with ametallic surface, such as a plated surface, can also be used to launchcircular polarized pulses and be within the intended scope of thepresent invention. As those of ordinary skill in the pertinent art willunderstand, the embodiments illustrated in FIGS. 9A-9B can be combinedwith reflector embodiments previously described herein and still be wellwithin the intended scope of the present invention.

Turning now to FIG. 10, illustrated is a SAW tag 1000 embodiment wherethe response to an interrogation pulse can be turned off. A detaileddescription of SAW tags 1000 of the type illustrated is set forth inHartmann One. The illustrated SAW tag 1000 has a substrate 1010 with apair of transducers 1020 located thereon. As described below, dependingon the phase of the transducers 1020 relative to one another, a SAW tag1000 response can be turned off or on when interrogated by a right orleft hand circular polarized interrogation pulse. Such a SAW tag 1000can be usefully employed with interrogation pulse focusing devices ofthe type illustrated in FIGS. 9A-9B.

In one embodiment of the invention the phase centers of the twotransducers 1020 are separated by (2n+½)π where n is an integer. Inphase and quadrature antennas 1030 on the SAW tag 1000 that receiveinterrogation pulses directed to the transducers 1020 are oriented inpositions 90° relative to each other. This means that circular polarizedsignals incident on such antennas 1030 will be in-phase in one directionand out-of-phase in the other. Each transducer 1020 will receive andgenerate a SAW only when it receives an in-phase signal. This feature isused to generate a unidirectional SAW that can be advantageously used toswitch a SAW tag 1000 off. The “switch” that shuts the SAW tag 1000 downin the embodiment illustrated is an absorbing reflector 1040 thatabsorbs the SAW and does not return a response. The absorbing reflector1040 and transducers 1020 can be constructed to absorb either right orleft hand polarized interrogation pulses. Thus, if the SAW tag 1000 isconstructed to absorb a left hand polarized interrogation pulses, itwill only launch a SAW generating a response when a right hand polarizedpulse is received. In short, the SAW tag 1000 is turned off when a lefthand polarized pulse is received because the SAW from the transducer1020 that received the left hand polarized pulse is absorbed by theabsorbing reflector 1040.

The present invention also provides a method of operating a system foravoiding code collisions from multiple SAW identification tags. In oneembodiment the method provides for focusing an interrogation pulse towithin a defined space and then discriminating between the codedresponses returned from tags located within that defined space. Suchembodiment will be clear to those of ordinary skill in the pertinent artfrom the detailed description of the system itself. The additionalembodiments of a method of operating the system described herein willlikewise be clear to those of ordinary skill in the pertinent art basedon the descriptions set forth.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A system for avoiding code collisions from multiple SAWidentification tags, comprising: a focusing means for focusing aninterrogation pulse by concentrating said interrogation pulse to withina defined space; and a discrimination means for discriminating betweencoded responses returned from tags located within said defined space. 2.The system as described in claim 1 wherein beam steering is used tofocus said interrogation pulse within said defined space.
 3. The systemas described in claim 1 wherein an antenna is used to focus saidinterrogation pulse within said defined space.
 4. The system asdescribed in claim 3 wherein said antenna is a parallel conductor pair.5. The system as described in claim 3 wherein said antenna is a helicalantenna.
 6. The system as described in claim 3 wherein said antenna is adielectric waveguide antenna.
 7. The system as described in claim 3further comprising a reflector coupled to said antenna.
 8. The system asdescribed in claim 7 wherein said reflector is an elliptical trough. 9.The system as described in claim 7 wherein said reflector is selectedfrom the group consisting of: a circular reflector; a curved reflector;a parabolic reflector; a re-entrant cavity; and an elliptical reflector.10. The system as described in claim 1 wherein said interrogation pulseis focused within said defined space by a waveguide.
 11. The system asdescribed in claim 10 wherein said waveguide projects a circularpolarized beam.
 12. The system as described in claim 10 wherein saidwaveguide has a first feed of about one-quarter wave length positionedat about a 90° position relative to a second feed of about one-quarterwave length.
 13. The system as described in claim 10 wherein a reflectoris coupled to said waveguide.
 14. The system as described in claim 13wherein said reflector is selected from the group consisting of: acircular reflector; a curved reflector; a parabolic reflector; are-entrant cavity; and an elliptical reflector.
 15. A method foroperating a system for avoiding code collisions from multiple SAWidentification tags, comprising: focusing an interrogation pulse toconcentrate said interrogation pulse to within a defined space; anddiscriminating between coded responses returned from tags located withinsaid defined space.
 16. The method as described in claim 15 wherein saidfocusing uses beam steering.
 17. The method as described in claim 15wherein said focusing uses an antenna.
 18. The method as described inclaim 17 wherein said antenna is a parallel conductor pair.
 19. Themethod as described in claim 17 wherein said antenna is a helicalantenna.
 20. The method as described in claim 17 wherein said antenna isa dielectric antenna.
 21. The method as described in claim 17 furthercomprising a reflector coupled to said antenna.
 22. The method asdescribed in claim 21 wherein said reflector is an elliptical trough.23. The method as described in claim 21 wherein said reflector isselected from the group consisting of: a circular reflector; a curvedreflector; a parabolic reflector; a re-entrant cavity; and an ellipticalreflector.
 24. The method as described in claim 15 wherein said focusinguses a waveguide.
 25. The method as described in claim 24 wherein saidwaveguide projects a circular polarized beam.
 26. The method asdescribed in claim 24 wherein said waveguide has a first feed of aboutone-quarter wave length positioned at about a 90° position relative to asecond feed of about one-quarter wave length.
 27. The method asdescribed in claim 24 wherein a reflector is coupled to said waveguide.28. The method as described in claim 27 wherein said reflector isselected from the group consisting of: a circular reflector; a curvedreflector; a parabolic reflector; a re-entrant cavity; and an ellipticalreflector. 29-41. (canceled)